Customers use computers at an internet cafe in Taiyuan, China. Photograph: Reuters

The carbon footprint of the internet:Around 300 million tonnes of CO2 per year, equivalent to every person in the UK flying to America and back twice over.

All carbon footprints are hard or impossible to pin down accurately, but the internet is a particularly complex case. This isn't just due to the fact that the "net" consists of millions or even billions of machines owned by countless people and companies. There's also another problem: even if we knew exactly how much energy all these devices consumed (which we don't), we still wouldn't know how much of that energy was spent on offline jobs (such as creating documents in Microsoft Office) and how much was spent on online jobs (such as emailing those documents to a friend or colleague).

It's possible, nonetheless, to take a rough stab at working out the internet's carbon footprint. A good place to start is the world's data centres – buildings packed top to bottom with servers full of the web pages, databases, online applications and downloadable files that make the modern online experience possible. Data centres use lots of electricity, both for powering the machines they contain and – all importantly – for the air conditioning needed to keep the servers from overheating.

According to a report by Gartner, data centres already account for around a quarter of the energy consumed (and the carbon emitted) by the information and communication technology (ICT) sector as a whole. In other words, around half a percent of global CO2 emissions.

By Gartner's figures, the world's PCs and monitors are even more power hungry, accounting for around 40% of the total ICT energy demand and 0.8% of global CO2 emissions. If we decided (somewhat arbitrarily) that half of the emissions from all these laptop and desktop machines were down to internet-based activity, and then add on the emissions from the data centres that make all this online activity possible, then the internet would clock in at around 1% of all the CO2 emissions released from burning fossil fuels. Put another way, the internet releases around 300m tonnes of CO2 – as much as all the coal, oil and gas burned in Turkey or Poland in one year, or more than half of those burned in the UK.

These figures tie in fairly well with a study by the UK's market transformation programme, which concluded that 343.5 million tonnes of CO2 was down to consumer and commercial ICT in 2005 – equivalent to around 1.2% of current fossil-fuel CO2 emissions. ICT's footprint is due to climb by 60% by 2030, the same report suggests. If that was to happen, and global emissions had already fallen by then, as climate change experts suggest they must, then the internet's share of total carbon output would climb significantly higher still.

In the meantime, it's interesting to note that 1% is about the same proportion as printing and paper-based publishing represents in the UK. The comparison isn't entirely valid, for a whole host of reasons, but the fact remains that despite ecological claims for the virtual economy, the digital era may be no less energy-hungry than the paper-based world of 20 years ago. Part of the reason is the so-called rebound effect – the phenomenon that when something (in this case the storing and interrogation of data) becomes cheaper and more energy-efficient, we often end up simply doing more of it, with the result that there is no net reduction, or even a rise, in cost or impact.

On the other hand, the internet is likely to be crucial to any move to a low-carbon world. Without its capacity to carry the huge flows of energy data, there could be no "smart grid", for example, and without online video conferencing it would be much harder to reduce the number of business flights in coming years. Ultimately, then, it's not just technological developments that will affect the growing carbon footprint of the Internet. Just as important is how we choose to use it.